A muffler, or exhaust device, used for a straddle-type vehicle (such as a motorcycle), has opposing design requirements. A muffler or exhaust device is required to effectively exhaust gas from an engine with high efficiency (that is, exhibit high exhaust efficiency) and simultaneously reduce or deaden exhaust sound caused by the exhaust gas that has been brought to high pressure and high temperature.
In recent years, noise regulations have been tightened and hence the need for reducing or deadening sound has been increased. Thus, it has been desired to maintain exhaust efficiency while reducing or further deadening the exhaust sound. One example of a muffler for a motorcycle that attempts to address the competing requirements of exhaust efficiency and sound deadening is described in Japanese Examined Utility Model Publication NO. 59-43455.
When the design of a muffler is considered in terms of only exhaust efficiency, it is most desirable that the muffler (or exhaust system) be kept as straight as possible. However, when the muffler is extended straightly, the muffler cannot be housed in the vehicle body of a motorcycle. Thus, to reduce resistance to exhaust, the muffler is designed to be extended towards the back of the vehicle body with as subtle of bends as possible. However, in reality, the design of the muffler in this manner is made difficult in many cases because of the connection with the front wheel and the consideration of the bank angle. Usually, a muffler having an ideal length in terms of engine performance, is very difficult to house in the body of the motorcycle without being changed in shape. Thus, the designing of a muffler having a length as close to the best length in terms of performance as possible, which keeps as smooth a shape as possible, and which is housed within the body of a motorcycle, involves an extensive design process, as compared with the designing of a muffler for a four-wheel passenger car.
Moreover, in addition to the exhaust efficiency, the weight of the muffler is an important design criteria. A motorcycle typically has a light vehicle body, and hence even a weight increase of 1 kg may have a large effect on the drivability of the motorcycle. Further, in addition to the weight of the muffler, arranging of the center of gravity of the muffler at a remote position will have a bad effect on the drivability of the motorcycle.
It is difficult, however, to reduce the weight of a motorcycle muffler because no matter how skillfully the structure of a muffler is designed, the muffler is required to have a certain amount of volume in order to enhance the effect of sound deadening. In many cases, when a muffler is adapted to stricter noise regulation requirements, the muffler needs to be enlarged in size, thereby increasing its weight.
If the metal plate used in construction of the muffler is made thinner to offset the weight increase, the metal plate will vibrate and cause large noises. As a result, mufflers of enlarged size tend to have increased weight, which in turn impairs the drivability of the motorcycle.
In this manner, the structure of the motorcycle is determined in consideration of various opposing design factors, so that it is extremely difficult to realize a muffler that satisfies exhaust efficiency and sound deadening characteristics and which has a reduced size.